Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
One type of photodetector that may be used in an image sensor or in a light detector is a single-photon avalanche diode (SPAD). A SPAD (also referred to as a Geiger-mode avalanche photodiode (G-APD)) is a solid-state photodetector capable of detecting a low intensity signal, such as low as a single photon. SPAD imaging sensors are semiconductor photosensitive devices made up of an array of SPAD regions that are fabricated on a silicon substrate. The SPAD regions produce an output pulse when struck by a photon. The SPAD regions have a p-n junction that is reverse biased above the breakdown voltage such that a single photo-generated carrier can trigger an avalanche multiplication process that causes current at the output of the photon detection cell to reach its final value quickly. This avalanche current continues until a quenching element is used to quench the avalanche process by reducing the bias voltage. The intensity of the photon signal received by the image sensor is obtained by counting the number of these output pulses within a window of time. Thus, one or more counters may be included in the readout circuitry of the imaging sensor.
However, conventional SPAD imaging sensors have a limited fill-factor because the counters occupy valuable space on the semiconductor substrate. Furthermore, formation of SPADs in a traditional CMOS process results in undesirable trade-offs having to be made between SPAD performance and transistor performance.